The present invention relates to a multilayer printed-circuit substrate having a plurality of electrically conductive circuits laminated one on one through insulating resin layers interposed therebetween, and the process of producing such a multilayer printed-circuit substrate. The present invention also relates to a wiring substrate or a multilayer wiring substrate which is light in weight and which is excellent in insulation property and plastic working property such as bending, drawing and the like, and a process of producing the same.
In recent years, various kinds of electronic appliances have been improved in reduction of size, reduction of thickness and the like. With the improvements, increase of density and increase of layers have been demanded of printed-circuit substrates used in the electronic appliances. Therefore, various multilayer printed-circuit substrates have been proposed.
Such a multilayer printed-circuit substrate has a structure in which hard insulating sheets of glass epoxy, polyimide or the like forming electrically conductive circuits are stuck on each other and in which the electrically conductive circuits are connected to each other by through-hole plating, or has a structure in which a plurality of electrically conductive circuits are laminated one on one through insulating resin layers made of thermosetting resin such as polyimide resin, epoxy resin, melamine resin, urea resin or the like on the surface of a hard sheet-shaped reinforcement material and in which a protective resin layer may be provided to protect the uppermost-layer electrically conductive circuit if necessary.
On the other hand, the following methods are known as methods for producing such a multilayer printed-circuit substrate.
The first method is a method comprising the steps of: providing an insulating layer made of epoxy resin or the like on a metal sheet; laminating a first-layer electrically conductive circuit on the insulating layer, the circuit being made of copper foil or the like and formed to be a predetermined circuit pattern by etching or any other method; laminating an insulating layer having through holes on the first-layer electrically conductive circuit by solder resist printing or any other method; and forming a second-layer electrically conductive circuit by printing a predetermined circuit pattern on the insulating layer by use of electrically conductive paste or the like prepared by mixing carbon powder, copper powder, silver powder or the like, and at the same time, electrically connecting the first-layer and second-layer electrically conductive circuits to each other through the electrically conductive paste filled in the through holes of the insulating layer to thereby attain two layers.
The second method is a method for producing a so-called metal core substrate which comprises the steps of: applying perforating treatment to a metal sheet to thereby form through holes; coating the whole of the metal sheet, inclusive of the through holes, with an insulating layer made of epoxy resin or the like; and forming upper- and lower-surface electrically conductive circuits on the upper and lower surfaces of the insulating layer by plating or any other method, and, at the same time, electrically connecting the upper- and lower-surface electrically conductive circuits to each other through the through holes.
The third method is a method which comprises the steps of: laminating a first-layer electrically conductive circuit made of copper foil or the like on a side of an insulating sheet of polyimide resin, polyester resin or the like having heat resistance and insulating property; laminating an adhesive sheet on the other side of the insulating sheet; forming through holes for forming electrically conductive portions and feedthrough holes for mounting electronic parts; piling up and pressure-sticking the thus prepared laminate on a hard substrate of paper phenol, glass epoxy or the like having the first-layer electrically conductive circuit laminated through the adhesive sheet; electrically connecting the first- and second-layer electrically conductive circuits to each other through the through holes by soldering or any other method; and forming a cover layer on the not-yet-soldered portions by solder resist coat printing or any other method.
The fourth method is a method for producing a multilayer printed-circuit substrate having four-layer electrically conductive circuits, which comprises the steps of: laminating metal layers of copper foil or the like on the upper and lower sides of an insulating sheet of glass fiber fabric or the like impregnated with epoxy resin varnish or the like; forming feedthrough holes for performing positioning; forming at the same time electrically conductive circuits on the upper and lower sides thereof by an ordinary method to thereby prepare a double-side laminate; forming single-side laminates each having a metal layer on one side of an insulating sheet; pressure-sticking the single-side laminates on the double-side laminate through prepregs under application of heat and pressure to thereby increase the number of layers; forming feedthrough holes in the thus prepared multilayer matter; and forming electrically conductive circuits by applying through-hole plating to the feedthrough holes and the metal layers of the single-side laminates and by etching the metal layers to form predetermined circuit patterns.
However, the first method has a problem in which the electrically conductive paste is so inferior in moisture-resisting and heat-resisting property that the resistance value of the electrically conductive circuits varies widely corresponding to the change of the outside temperature and humidity, and in which it is difficult to make the substrate fine so that reliability deteriorates when long electrically conductive circuits are formed.
The second method has a problem in which the steps for forming the insulting layers and electrically conductive circuits are so complex that producing cost becomes high.
The third method has a problem in which the producing steps are so complex that cost increases because the sticking step using the adhesive sheet and the handwork step for soldering are required, and in which when an easy pressure-sensitive adhesion type adhesive sheet is used as the adhesive sheet, peeling of the adhesive portions and cracking of the solder connection portions occur so easily that reliability deteriorates.
The fourth method has a problem in which the epoxy resin used is so inferior in heat-resisting property that a so-called smear phenomenon that the epoxy resin is softened by heat at the time of forming feedthrough holes by using a drill or the like and enters into the feedthrough holes to cover the cutting surfaces of the metal layers occurs and, accordingly, connection between the respective electrically conductive circuits with the use of the cutting surfaces of the metal layers exposed into the feedthrough holes by through-hole plating means becomes insufficient or a so-called internal layer peeling phenomenon occurs to increase the number of defective products.
Therefore, the inventors have proposed a multilayer printed-circuit substrate and a method of producing the same to solve the aforementioned problems. The proposed multilayer printed-circuit substrate has a hard sheet-shaped reinforcement material, a plurality of insulating resin layers laminated on a surface of the hard material and hardened at the time of pressure-sticking under application of heat and pressure, a plurality of metal-made electrically conductive circuits laminated between the insulating resin layers and on the uppermost thereof and etched to predetermined circuit patterns, and electrically conductive portions for electrically connecting adjacent upper- and lower-positioned electrically conductive circuits to each other by filling an electrically conductive matter in holes formed to pass through the upper-positioned electrically conductive circuits at predetermined positions between the adjacent upper- and lower-positioned electrically conductive circuits and pass through the insulating resin layers located between the upper- and lower-positioned electrically conductive circuits. The proposed method for producing such a printed-circuit substrate is a method of laminating a plurality of electrically conductive circuits through insulating layers on a hard sheet-shaped reinforcement material, which comprises the steps of: forming a half-hardened insulating resin sheet having metal-made electrically conductive circuits laminated on the upper surface thereof and showing a stable half-hardened condition; laminating the half-hardened insulating resin sheet on lower-positioned electrically conductive circuits laminated through a hard sheet-shaped reinforcement material or through insulating resin layers hardened at the time of pressure-sticking under application of heat and pressure; hardening the half-hardened insulating resin of the half-hardened insulating resin sheet by pressure-sticking under application of heat and pressure; and forming electrically conductive portions for electrically connecting adjacent upper- and lower-positioned electrically conductive circuits to each other by filling an electrically conductive matter in feedthrough holes formed in the upper-positioned electrically conductive circuits and the insulating layers to reach the lower-positioned electrically conductive circuits (Japanese Patent Application Nos. 62-246,892 and 62-246,893). According to the proposed printed-circuit substrate and the method of producing the same, the thus prepared printed- circuit substrate is excellent in heat-resisting property and durability against solvent and excellent in reliability because the failure of electrical connection caused by the smear phenomenon at the time of through-hole plating little occurs. Further, the producing steps are simplified to attain saving of producing cost.
However, in the proposed multilayer printed-circuit substrate, irregularity may occur in adhesion between the respective electrically conductive circuit and the insulating resin layer covering thereon though the degree of irregularity varies according to the kind of the metal forming the electrically conductive circuit and the kind of the resin forming the insulating resin layer. Accordingly, in the case where heat stress caused by soldering or the like acts or in the case where heat stress caused by the exchange of parts acts repeatedly, swelling or peeling may occur between the electrically conductive circuit and the insulating resin layer covering thereon. Accordingly, failure of electric conduction may occur. In particular, in the case where the exchange of parts is repeated, reliability may deteriorate.
Further, the aforementioned problems arise not only in the multilayer printed-circuit substrate having such stiffness but in the case where a flexible multilayer printed-circuit substrate having flexibility such as for example two-layer structure designed to attain high density to put electronic parts in the limited space of electronic appliances in which both reduction of size and reduction of thickness are required is bent in practical use.
In short, the conventional flexible multilayer printed-circuit substrate has a structure, as shown in FIG. 1, substantially constituted by a lower-positioned laminate X and an upper- positioned laminate Y, the lower-positioned laminate X being formed by sticking copper foil to a flexible insulating sheet 101a, such as a polyester sheet, a polyimide sheet or a glass epoxy sheet having a thickness of 0.2 mm or less, and then applying an etching treatment to the copper foil to prepare an electrically conductive circuit 102a, the upper-positioned laminate Y being formed to have an insulating sheet 101b and an electrically conductive circuit 102b in the same manner as the lower-positioned laminate, the upper-positioned laminate Y being provided with feedthrough holes 103 formed by applying a perforating treatment to a predetermined position of the upper-positioned laminate Y, the upper-positioned laminate Y being laminated onto the lower-positioned laminate X by heat-pressing means or the like through an adhesive agent layer made of a thermosetting adhesive agent such as an epoxy resin or the like or a thermoplastic adhesive agent such as an acrylic resin, the electrically conductive circuits 102a and 102b of the upper- and lower-positioned laminates X and Y being electrically connected to each other by using the feedthrough holes 103 formed in the upper-positioned laminate Y, the upper-positioned laminate being provided with a protective film 106 formed by applying a so-called cover layer, such as a film or a solder resist, of the same material as that of the insulating sheets 101a and 101b to other portions of the upper- positioned laminate which are not soldered.
However, in the thus prepared flexible multilayer printed-circuit substrate, the laminates B1 and B2 are laminated through the adhesive agent layer 104. Accordingly, in the case where the adhesive agent layer 104 is formed of a thermosetting adhesive agent, cost required for the adhesive agent is high. Further, a problem arises in that the step of application of the adhesive agent requires much labor. In the case where the adhesive agent layer 104 is formed of a thermoplastic adhesive agent, the adhesive agent is so inferior in heat-resisting property that cracking is apt to occur in the solder portions. Therefore, the soldering step must be conducted by handwork in order to prevent the lowering of reliability. Further, a problem arises in that the adhesive property thereof is so inferior that peeling is apt to occur. In any case, a problem arises in that the producing cost is too high to produce a reliable flexible multilayer printed-circuit substrate.
On the other hand, as a base material of a wiring substrate used in various electronic appliances, heretofore, rigid insulating sheets having stiffness, such as glass, ceramics, glass epoxy, and the like, have been generally used. In general, this type wiring substrate is shaped like a flat sheet and used after being put in a substrate packing space formed in the inside of electronic appliances.
However, such a wiring substrate is inferior in plasticity. Therefore, in the case such a wiring substrate cannot be used in the form of a single flat substrate for some reason or more of the substrate packing space provided in electronic appliances, it is necessary to employ a structure of divisional connection in which the substrate is divided into several members which are connected by use of cables or connectors. This type wiring substrate has a problem in that the number of parts is inevitably increased, and, accordingly, this type wiring substrate cannot sufficiently cope with the demands for reduction of size, reduction of weight, reduction of thickness, and the like, in some electronic appliances.
In recent years, the following wiring substrates have been proposed to cope with the recent demands for reduction of size, reduction of thickness, and the like. One of the proposed wiring substrates is a wiring substrate called "a metal base substrate" which has a metal sheet 201 such as an aluminum sheet, a steel sheet, or the like used as a base material, and an electrically conductive circuit 203 laminated on the metal sheet through an insulating layer 202 of insulating resin, such as epoxy resin, glass epoxy, polyimide, polyamide, or the like, and formed by applying etching treatment to copper foil or the like to attain a predetermined circuit pattern, as shown in FIG. 2. Another one of the proposed wiring substrates is a wiring substrate called "a metal core substrate" which is prepared by the steps of: forming through holes 204 in a metal sheet 201; coating the whole surface, inclusive of the inside of the through holes 204, with an insulating layer 202 of insulating resin such as epoxy resin or the like to prepare through holes 105 coated with the insulating layer 202; and forming an electrically connection portion 206 for connecting between electrically conductive circuits 203a and 203b laminated on the double sides of the metal sheet through the insulating layer 202, as shown in FIG. 3.
Such a wiring substrate having the metal sheet 201 as a base material has an advantage in that the degree of freedom in the formation of the substrate packing space provided in electronic appliances increases, because the wiring substrate is excellent in strength and plastic working property so that the the wiring substrate can be suitably bent or drawn so as to have a predetermined shape in accordance with the requirement of electronic appliances.
Even in such a wiring substrate having the metal sheet 201 as a base material, there is a problem in that corner portions of the insulating layer 202 are apt to be injured at the time of plastic work or practical use to thereby lower reliability upon insulation. Furthermore, in the case of the metal core substrate, the whole surface inclusive of the inside of the through holes 204 must be entirely coated with the insulating layer 202. There arises a problem in that much labor is required for the coating work.
Accordingly, the metal base substrate and the metal core substrate have, limitation in that no portion except non-wiring portions can be subjected to plastic work. Furthermore, it is difficult to make the substrates have a multilayer structure for the purpose of high density. In short, it cannot be said that the conventional substrates are sufficient for development in reduction of size, reduction of thickness, reduction of weight, and the like, of electronic appliances.